Electronic circuits comprise amongst others frequency converters which are used in electro-mechanical drive systems for generating, from an alternating current (AC), an alternating current with varying frequency and amplitude for the direct power supply of electrical machines, as for example, three-phase motors.
Although other configurations are also known in prior art, typically, such frequency converters consist of an input circuit for rectifying a mains AC current, an intermediate circuit, and a power circuit comprising semiconductor switches which generate the power supply having a required frequency and amplitude.
Further, a capacitor is implemented in such frequency converters which actually represents the power storage and which is connected downstream of the rectifier. It is also known in prior art to provide a voltage limiter in the intermediate circuit, wherein the higher a voltage applied to the electronic circuit, the lower its resistance will become. However, such a voltage limiter is only effective with high voltages above 450 Volt, and its response time is rather slow.
Moreover, it is known in prior art to limit the current in the intermediate circuit to a predetermined maximum inrush limit, when the intermediate circuit capacitor is being charged. This typically is achieved by a resistor having a high resistance. Due to the losses entailed by implementing the above mentioned resistor with high resistance, the latter is only activated in case it is actually needed. Otherwise, it is always short-circuited. The short-circuit is effected by an electronic switch element, the switch element could advantageously be a gated gallium nitride or silicon based semiconductor device with a mainly resistive ON characteristic as for example, a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor), which short-circuits the resistor depending on the current.
Most MOSFETS, however, although being rather inexpensive, are only able to operate in linear mode for a short period of time. Therefore, the voltage over the switch element has to be measured. If the voltage rises above a certain threshold voltage value, then the switch has to open, because of protection of the switch element.